1. Field of the Invention
The invention relates to technology for detecting a failure in an automatic transmission, and more specifically, to technology for accurately detecting a cause of an increase in engine speed while running.
2. Description of the Related Art
An apparatus which detects a failure while running is typically provided in an automatic transmission mounted in a vehicle. The failure to be detected is, for example, an electrical failure such as a failure of a sensor or an actuator or the like, or a mechanical failure such as sticking of a shift valve or a shift solenoid, or slipping of a frictional engaging element. When these types of failures occur, a safety device activates and notifies the driver of the failure. The location of the failure can then be confirmed and the necessary repairs can be made.
Japanese Patent Laid-Open Publication No. 1-172663, for example, discloses a failsafe control device for an electronically controlled automatic transmission, which detects a failure of a structural part of the automatic transmission. This control device includes a transmission input speed sensor, a circuit which calculates an actual gear ratio of the transmission based on one of a transmission input speed, a transmission output speed, and a vehicle speed, a circuit which obtains a regular gear ratio, a circuit which compares the regular gear ratio with the calculated gear ratio, and a circuit which detects a failure of a structural part of the automatic transmission based on the comparative results.
The failsafe control device calculates the actual gear ratio of the automatic transmission based on one of the detected input speed, output speed, and vehicle speed. On the other hand, the regular gear ratio is calculated based on a shift lever position and an output state of a shift solenoid, for example. Therefore, when the difference between the actual gear ratio and the regular gear ratio deviates from a preset value, a failure is detected in the automatic transmission.
When the automatic transmission has been instructed to shift to a predetermined gear speed, two kinds of failures may occur: a failure in which the transmission of torque is interrupted (hereinafter referred to as “first failure”) and a failure in which application of the frictional engaging element is insufficient such that the frictional engaging element slips (hereinafter referred to as “second failure”).
The first failure occurs due to an abnormality in the control valve which directly controls the hydraulic pressure supplied to the frictional engaging element with which the predetermined gear speed is established. In this case, because the frictional engaging element is not fully applied, torque may not be transmitted, which may result in the transmission being in a neutral state. At this time the speed of the power source increases because there is almost no load on the power source. On the other hand, the second failure occurs due to a failure of a regulator valve which regulates the hydraulic fluid discharged from an oil pump. When the regulator valve fails, the base pressure (so-called line pressure) of the hydraulic fluid of the automatic transmission decreases. As a result, the hydraulic pressure supplied to the frictional engaging element also decreases. In this case, the amount of torque (i.e., torque capacity) able to be transmitted by the frictional engaging element decreases. As a result, only a portion of the torque input to the automatic transmission is transmitted, which may result in an increase in speed of the power source.
While it is possible for the control device to detect whether there is a failure in the automatic transmission, it is not easy to determine the cause of that failure when there are conceivably two or more causes. That is, when the speed of the power source increases while the vehicle is running in a predetermined gear speed, it is not easy to identify a state of the automatic transmission which could bring about such an increase.